The present invention relates to processing sound signals having spatialised components which create a multidimensional environment for the sound. Further, the present invention relates to improving the reproduction of binaural (two channel) sound, particularly when it is desired to give a listener an impression of virtual sound sources being located some distance away from the listener.
For a general reference to the field and on the problems associated with reproduction of sound having spatial components, reference is made to xe2x80x9cA 3-D Sound Primer: directional hearing and stereo reproductionxe2x80x9d by Gary S Kendall appearing in the Computer Music Journal, 19;4 at pages 23-46, Winter 1995.
Methods are generally known for the generation of binaural sound where headtracking of the listener""s head movements is utilised to modify the processed output to provide a better impression of sound located some distance away from the listener. These methods include:
1. The xe2x80x9cHeadscapexe2x80x9d program utilised in conjunction with the Huron Digital Audio Convolution Work Station both of which are available from the present assignee Lake DSP Pty Ltd and rely upon the smooth switching between pre-computed FIR filter responses in response to a listener""s head turning.
2. U.S. patent application Ser. No. 08/723,614 filed Oct. 2, 1996 in the name of the present applicant and inventor and entitled xe2x80x9cMethods and Apparatus for Processing Spatialised Audioxe2x80x9d which describes a method for headtracked playback of B format xe2x80x9cambisonicxe2x80x9d sound fields.
3. Existing products from other manufacturers which utilise rapidly changing head related transfer function (HRTF) filters to perform headtracked playback of binaural sound.
Each of these systems rely on arrangement similar to that disclosed in FIG. 1 herein in that a listener 2 utilises a pair of headphones 3 having an integrally mounted headtracking means 4 which tracks the orientation of the user""s head 2. The headtracking means 4 is normally in communications with a headtracking unit 5 which continuously determines a current orientation of the user""s head. This information 6 is output to the binaural processing system 7 which manipulates a series of audio inputs 8 to produce corresponding right 10 and left 11 output sound channels for playback to the user""s head 3.
The disadvantage of the arrangement 1 of FIG. 1 is that a headtracking unit eg. 4, 5 must be provided and this adds a large degree of complexity and expensive to the arrangement 1. Further, most headphones in use today do not have any headtracking facility but are rather stereotype devices.
The arrangement 1 is primarily concerned with audio processing the input signals 8 so that there is an altering of corresponding outputs 10, 11 in response to the turning of the listener""s head 2. This is provided as a means to create a more stable audio sound field so that the location of the virtual sounds around the listener do not change when a listener turns his/her head. Additionally, the audio processing systems generally provide a better illusion of sounds in front of the listener. Tracking the rotation of a listener""s head greatly enhances the impression of frontal sounds, defeating the front-back confusion that commonly occurs with binaural sound and is a well known problem with the prior art.
It is an object of the present invention to provide an improved means of front-back discrimination of sounds to the listener without the need for the provision of an expensive headtracking unit. The removal of the headtracking from the playback process also has the advantageous effect of allowing a binaural signal, such as a stereo pair, to be utilised by one or more listeners without the need for any additional processing at the time of playback.
In accordance with the first aspect of the present invention there is provided a method for creating a multichannel audio signal that provides the impression of spatial sound, said method comprising:
providing an expected multichannel audio output signal having spatialised soundfield components;
perturbing said spatial components; and
utilising said perturbed spatial components to determine said multichannel audio signal.
Preferably, the multichannel audio signal comprises two channels adapted for playback over headphones. Further, the perturbations preferably comprise a series of substantially random rotations, substantially in the horizontal plane. Further, the perturbations can be substantially in accordance with the expected head movements of listeners to the audio signal.
Methods disclosed include methods for deriving expected head movements from a group of listeners to the audio signal and subsequently using these movements with like audiences. As a further refinement, a random movement can be added to the expected head movement.
Preferably the invention works with large scale movements of sound sources and, as a refinement, the perturberances can be created such as to not incorporate any change in arrival time of simulated acoustic arrival times.
There is also a disclosed an apparatus for implementing the invention by means of a DSP arrangement or the like.